1. Field of the Invention
The present invention relates to a titanylphthalocyanine crystal and a method of producing the titanylphthalocyanine crystal, and to an electrophotographic photoreceptor, method, apparatus and process cartridge using the titanylphthalocyanine crystal.
2. Discussion of the Background
An organic pigment has been used as a filler for a coating material since comparatively a long ago. In particular, the organic pigment has the advantage over an inorganic pigment in its abundance of color variation. Further, recently as an application of the organic pigment, a variety of materials have been produced because it was spotlighted as a material for an organic photoelectric transfer device.
Almost all methods of forming a film including the organic pigment are wet coating methods. It is not exaggerated to say quality of a film coated by the wet coating methods depends on quality of a dispersion liquid including the pigment. Dispersibility of the pigment in the dispersion liquid is one of essentials for the quality of the dispersion liquid. Therefore, a good dispersion liquid includes the pigment sufficiently dispersed in a vehicle and continues the dispersed status for a long term.
Recently, digital electrophotographic methods prevail, and particularly a negative and positive development (reverse development) is mostly used. In a case of the negative and positive development, a part of a photoreceptor used in the electrophotographic method is irradiated by imagewise light and a potential of the part decreases to form a toner image thereon. This is performed in consideration of a longevity of a light source and fatigue of the photoreceptor because writing ratio of an original copy is more or less 10% at most. However, background of a produced copy corresponds to a non-irradiated part (high potential part) of the photoreceptor, and when a photosensitive layer (particularly a charge generation layer) has a coating defect, a background portion (white background portion) of the photoreceptor which should have maintained potential decreases potential, resulting in occasional occurrence of point defects such as background fouling and black spots. These defects are occasionally mistaken for points in drawings and periods and commas in English drafts, and it can be said that the defects are fatal defects as images. Such point defects are often caused by a layer constituted of dispersed film including pigments, etc. Therefore, in order to decrease the point defects, a dispersion liquid having good dispersibility of pigments, etc. has to be produced. A particle size of the pigment in the dispersion liquid is preferably as small as possible, and a pigment having a primary particle size not greater than about 0.2 μm considerably decrease the point defects as mentioned above.
In order to produce such dispersion liquids, a variety of dispersers and dispersion systems and methods of increasing dispersibility have been suggested. For example, any of Japanese Laid-Open Patent Publications Nos. 4-373362, 5-188614, 7-289870, 8-44086, 8-123945, 8-272111, 9-211873, 11-30871, 11-258827, 2000-126638, 2000-181104, 2000-281931, 2001-265027 and 2001-290292 discloses a technology to miniaturize (reduce a particle size of) synthesized large average-size pigment particles in a dispersion liquid as small as possible using a variety of dispersers, dispersing conditions or improving the dispersers and conditions. It can be said that these dispersion methods are good technologies in terms of efficiently dispersing the pigment until the pigment has the primary particle size. However, it is quite difficult to reduce the particle size more than the primary particle size, and a limit of the particle size in the dispersion liquid basically depends on the primary particle size of the pigment. Methods of pulverizing the primary particle with an enormous energy are included in the methods, but these break the crystal and have problems such as deterioration of dispersion efficiency and transform of the crystal form as mentioned later.
On the other hand, as a useful charge generation material, titanylphthalocyanine is known. The titanylphthalocyanine is a polytypic crystal pigment including many crystal forms as an aggregate even when having the same formula. Particularly, a titanylphthalocyanine crystal having at least a maximum diffraction peak at of a Bragg (2θ) angle of 27.2±0.2° when irradiated with a specific X-ray of CuKα having a wavelength 1.542 Å has significantly a high photocarrier generation efficiency as a charge generation material for an electrophotographic photoreceptor among polytypic crystal pigments. However, the titanylphthalocyanine has a quasi-crystallinity and easily transforms to other crystal forms. When an excessive energy is applied to the crystal, the crystal easily transforms to a titanylphthalocyanine crystal having a maximum diffraction peak at Bragg (2θ) angle of 26.3°. Because this crystal has considerably a lower photocarrier generation efficiency than the above-mentioned crystal, when even a partly transformed crystal is used for a charge generation material for a photoreceptor, problems such as deterioration of photosensitivity and increase of residual potential in repeated use occur.
When the dispersion condition is tempered to prevent this crystal transform, the resultant dispersion liquid includes a pigment having a large particle diameter or remaining coarse pigment particles. When such a dispersion liquid including a pigment having a large average particle diameter is used, surface area of the total charge generation materials becomes small, and it becomes difficult for the material to give and receive a charge to and from charge transport material, resulting in problems such as deterioration of photosensitivity and increase of residual potential in repeated use of the resultant photoreceptor occur. When the coarse particles remain, the resultant image has problems such as background fouling and black spots occur in the negative and positive development.
As just described, in the dispersion of the titanylphthalocyanine pigment particles, stability of the crystal form and miniatulization of the particles have a trade-off relation and means to easily solve this problem have not been available.
As another method of reducing the particle size of the pigment in the dispersion liquid, a method of using a pigment which can easily be dispersed, i.e., which has significantly a small primary particle diameter can be considered. In this method, a pigment previously having a small primary particle diameter is synthesized to obtain a dispersion liquid including a pigment having a small particle diameter without applying an excessive energy to the pigment. This method not only improves dispersion efficiency but also can be considered as significantly an effective method when using a pigment in which a crystal easily transforms such as the above-mentioned titanylphthalocyanine.
However, approaches in terms of pigment synthesis have hardly been seen so far. Only Japanese Laid-Open Patent Publication No. 2000-239556 discloses a method of producing a dispersion liquid using means of combining a crystal conversion and dispersion means. This method can form a dispersion liquid including a pigment having a primary particle diameter, which is formed by the crystal conversion. However, considering the dispersion liquid is used for a coating liquid, a crystal conversion solvent is not limited to a suitable coating solvent and the method has a drawback of having a restriction in coating. In addition, because the dispersion liquid cannot be stored as a pigment powder, the method also has a drawback of having a restriction in storage.
Because of these reasons, a need exists for a titanylphthalocyanine crystal having a small primary particle size and a method of producing the titanylphthalocyanine crystal to prepare a dispersion liquid including a pigment having a small average particle size without a crystal transform of the titanylphthalocyanine crystal which is quite useful as a charge generation material for an electrophotographic photoreceptor.